1. Field of the Invention
This invention relates to the manufacture of paper towels and the like. More particularly, it relates to methods of embossing paper sheets.
2. Brief Description of the Prior Art
It is well known in the papermaking industry that embossing a paper web or sheet gives the sheet bulk, a property that is of particular importance in manufacturing paper towels because it is associated with softness and absorbency by the consumer. Bulk is not only important with regard to the feel of the individual sheets, but also in regard to the bulkiness of the packaged product. In the case of paper towels, where the product is packaged in rolls it is desirable to minimize nesting of the sheets within one another in order to maintain a larger roll diameter with a given number of sheets per roll.
In manufacturing multi-ply paper sheets, there are basically two processes which are well known in the art. One is a nested process and the other is a pillowed process. The nested process is illustrated in U.S. Pat. No. 3,694,300 to Small and U.S. Pat. No. 3,867,225 to Nystrand and generally involves the separate embossing of two plies between a rubber roll and a steel embossing roll, applying an adhesive with a roller to the protruding embossments of one of the plies, and combining the two plies together such that the protrusions of one sheet are attached within the depressions of the other. The pillowed process is similar and is illustrated by U.S. Pat. No. 3,961,119 to Thomas. In this process, however, the plies are laminated together such that the protrusions of one ply are not aligned within the depressions of the other.
The use of embossed designs to improve the aesthetic appeal of the product is also known in the art, as illustrated by U.S. D. No. 262,747 to Erickson which shows a shamrock design.
Regardless of the method used to emboss sheets, however, it is known that the embossing process causes a reduction in strength of the sheet due to breakage of inter fiber bonds. Because the tensile strength in the cross-machine direction (CD) of creped wadding sheets is lower than the machine direction (MD) tensile strength, CD strength is the limiting factor insofar as the overall strength of the sheet is concerned. As a result, CD strength reduction is a critical factor in the embossing process. Therefore there is a need for an embossing process which reduces the strength degradation, particularly in the CD of the sheet.